Hybrid electric vehicles (HEVs) and fully electric vehicles (EVs) are emerging as promising solutions for near-term sustainable transportation. The deleterious effects of conventional internal combustion engines (ICEs) on the environment, and certain economic issues associated with petroleum-based fuels are the major motivations in development of electric powertrains.
While EVs completely rely on the power supply from an electrochemical storage system (e.g., batteries), in HEVs a combination of ICE power and battery system power provides the propulsion in the hybrid drivetrain. Addition of a regeneration system to the vehicle allows recharging the batteries by capturing the kinetic energy during braking. Moreover, a small ICE can be used as a generator in EVs to recharge the batteries and extend the driving range.
Hybrid and fully electric vehicles have many hurdles to overcome when it comes to safety and efficiency concerns. Despite technological achievements in battery technology, large-scale application of high-energy and high-power batteries has not reached to its full potential. This shortcoming is associated with the fact that charge intake, power delivery characteristics, and calendar life of batteries strongly depends on their temperature. It is a well-evidenced fact that excessive heating of batteries during operation (charging and discharging) leads to imbalanced reactions, which consequently trigger serious safety issues such as fire and explosion. Moreover, exposure of batteries to sub-freezing temperatures drastically reduces their power delivery. Accordingly, battery thermal management system (BTMS) is a must for all large- and medium-scale battery packs to keep their temperature within an optimal range regardless of the load on the battery pack.
Lithium-ion (Li-ion) batteries have become the dominant battery technology due to several compelling features such as high power and energy densities, long cycle life, excellent storage capabilities, and memory-free recharge characteristics. Prismatic Li-ion cells, also known as pouch-shaped cells, are well known in the art, and are favored in automobiles electrification owing to the negligible weight for the case (pouch), relatively low manufacturing costs, and flexibility in shape design.
Lithium based batteries are room temperature batteries; this means that their ideal operating temperature is around 25° C. Nonetheless, they can operate within the range of −20° C. to 60° C., but at temperatures below 0° C. their capacity fades rapidly and at temperatures above 50° C. they become prone to serious thermal hazards. Accordingly, thermal management of Li-ion batteries is critical to promote their safety and performance.
In general, complexity of a BTMS increases with the size of a battery system. Significant temperature variations can occur between individual cells, as the size of battery system increases. If one cell is at a higher temperature compared to the other cells, its electrical performance will be different, and this leads to imbalance performance of the whole battery pack. Thus, to promote the peak performance, the differential temperature between the cells in the battery pack should be minimized; meanwhile the entire battery pack must be kept within a desired temperature range.
Multiple cell batteries, especially Lithium-ion (Li-ion) batteries are widely used due to its high energy density, high specific energy, long life cycle and flexible and lightweight design. Generally a multiple cell lithium-ion battery includes a battery case and a set of cells grouped in the battery case as battery modules. Each of these battery cells includes a positive electrode having a positive lead and a negative electrode having a negative lead. In construction these positive leads and negative leads are connected in series or in parallel, or in combination to meet different requirements of voltage and energy on battery level.
Normally a battery includes a bus bar, which is a metallic conductor having low impedance and high current carrying capacity and connects electrodes in the required arrangement. The bus bar is generally used as a conductor to distribute electric power to several points of the battery and the bus bar is generally used to connect electrodes of each battery cell where the bus bar connects the cells in series wherein the bus bar connects the positive terminal of one battery cell to a negative terminal of another battery cell. The bus bar is generally welded to each electrode terminal. Alternatively, bus bars may be connected to the electrode terminals by mechanical fasteners. Accordingly, the bus bars are used to carry substantial electric currents between cells over relatively short distances;
An example a traditional prior art bus bar arrangement for a lithium ion battery pack is shown in FIGS. 1A and 1B. As is known in the art, electrical transfer is the main purpose of the bus bars. However, as the length of a bus bar may become relatively long (as shown in FIGS. 1A and 1B), the temperature of the bus bar may increase as electricity travels through the bus bar. Accordingly, in conducting electrical currents between the battery cells, heat 174 may, but not necessarily, transfer from the center region 114 of each bus bar 112 (where heat accumulates between the cells) toward the end regions 116 of each bus bar 112 where the bus bar 112 is coupled to a cooling fin (not shown) via cell components. The end regions 116 of each bus bar may be affixed to each cell tab (electrode) so that the heat could be transferred from the cell cap 164 to the cell tab then to the cell and to a cooling fin which allows heat to be dissipated. It is understood that cables 120 may connect the battery pack to the vehicle.
However, given the rather long z-shaped bus bars, relatively high temperatures are generated as current travels through the bus bars 112. Moreover, it is difficult for the heat 174 to transfer from the bus bars 112 to the cooling fins given the rather long distance between the center of the z-shaped bus bars and the cooling fins.
Accordingly, there is a need for an improved bus bar arrangement for a lithium ion vehicle battery which better manages thermal energy.